The present invention relates to an offset printing machine and more particular to an offset printing machine in which an exact adjustment of image position may be effected for keeping a printing image position constant relative to a piece of fed paper.
With a variety of printing plates, including a paper printing for an offset printing plate machine or press, it is very difficult to position a pictorial image portion exactly at a predetermined position of the printing plate during a printing operation. It is inevitable that a small amount of displacement or shear in image position will be left between an original and the printing plate for every printing.
It is therefore necessary to keep constant a printing image position relative to the fed paper by adjusting the image position against the displacement of the printing portion or the displacement caused upon loading the printing plate onto a printing cylinder whenever the printing plate is replaced by another on the printing cylinder during the printing operation.
An example of such an offset printing machine is disclosed in Japanese Examined Patent Publication No. Hei 2-16215. The machine will be explained with reference to FIGS. 9 and 10 in which an adjustment gear 116 is mounted on a frame 113 supporting a first rubber blanket cylinder 101. The adjustment gear 116 is axially movable by a solenoid 114 and is drivingly rotatable by a gear 124 mounted on a shaft of a step motor 115. The rotation of the adjustment gear 116 is transmitted, through a gear 131 and a gear transmission mechanism 118 supported, through a bracket 117, by a cylinder gear 107, to a worm wheel 129 provided on the bracket 117.
The above described gear transmission mechanism 118 is constructed as follows. A spur gear 125 and a bevel gear 126 are disposed on a shaft 127 rotatably supported on the bracket 117 fixed to the cylinder gear 107. The worm gear 129 and a bevel gear 128 which is arranged perpendicular to the shaft 127 are disposed on a common shaft 130 rotatably supported on the bracket 117 but are not displaceable in the axial direction. Both the bevel gears 126 and 128 are engaged with each other. The spur gear 125 is engaged with the gear 131 which is engagable with the adjustment gear 116. The worm gear 129 is engaged with sector gear 119 fixed to one side of the rubber blanket cylinder 101. The sector gear 119 is rotated together with the rubber blanket cylinder 101.
The cylinder gear 107 is engaged with another cylinder gear 108 of a rubber blanket compression cylinder 103, so that both the rubber blanket and compression cylinders 101 and 103 are rotated in synchronism with each other.
As described above, an adjustment shaft 116a of the adjustment gear 116 passes through a hole 120a of a support member 120 fixed to the frame 113 so that the adjustment shaft 116a is rotatably slidable in the axial direction. The adjustment gear 116 is fixed to an inward end of the adjustment shaft 116a whereas the other end of the adjustment shaft 116a is coupled to an operating rod 122 supported on a bracket 121 fixed to the frame 113 and drivingly swung by the solenoid 114. A spring 123 is laid around the adjustment shaft 116a so that the adjustment gear 116 is out of engagement with the gear 131 of the above described gear transmission mechanism 118 during the non-operation period of the solenoid 114.
Upon the image position adjustment, the solenoid 114 is operative and the adjustment shaft 116a is pushed inwardly against the biasing force of the spring 123. As a result, the adjustment gear 116 is engaged with the gear 131 of the gear transmission mechanism 118. Under this condition, when the step motor 115 is driven, the rotation of the adjustment gear 116 is transmitted through the gear transmission mechanism 118 to the worm wheel 129 to thereby rotate the blanket gear 107 relative to the rubber blanket cylinder 101 for the adjustment of the image position.
However, in the conventional image adjusting device for the offset printing machine, in the case where the adjustment gear 116 is moved in the axial direction by the solenoid 114 and engaged with the gear 131 to adjust the image, there is a fear that the gear 116 would not be well engaged with the gear 131 due to, for example, an abutment between teeth of these gears even if crests of these teeth would be rounded or bevelled. In such a case, the motor 115 rotates, the gear 116 is rotated through the shaft 116a by the rotation of the motor 115, the crests of the gears 116 and 131 are moved out of abutment with each other, and then the crests and the troughs of these gears are engaged with each other. Through these operations, the rotation of the gear 116 is finally transmitted to the gear 131.
In this case, it should be noted that the motor 115 rotates by an amount sometimes greater than the amount corresponding to the actual image movement. Since a portion of the movement by the motor 115 is consumed before the gears 116 and 131 mesh with each other as described above, all of its rotation is not transmitted to the gear 131.
Further, a gear transmission mechanism 124 consisting of the gear 140 and a gear 150 engaged with the gear 140 for transmission of the rotation of the motor 115 to the shaft 116a suffers from a backlash. Thus, the exact rotational amount of the motor 115 is not always transmitted to the gear 116.
Therefore, the conventional image adjusting device suffers, mainly for the two reasons, from such a defect that the rotational amount of the motor which rotates in response to the number of pulses corresponding to the image position movement calculated and set in advance is not exactly transmitted to the gear 131 and the image position movement is not accurate.